Time-Frequency Analysis of Partial Discharge Current Pulses in Different Gas Environment under Lightning Impulse

This paper deals with a time-frequency analysis of the measured partial discharge (PD) currents in different insulation gases. These gas environments consist of a pure SF6 (sulphur-hexafluoride) and sulphur-hexafluoride and nitrogen (1 % SF6 + 99 % N2) mixture, under both positive and negative lightning impulse (LI) voltage stresses. In this study, the short time Fourier transform was used to extract the time-frequency information of PDs for different gases at different pressures, and these results were compared to each other. Thus, the relationship between the time, amplitude, and frequency of PD currents was studied. Moreover, some statistical formulas, such as mean, standard deviation, kurtosis, and skewness were applied to the time-dependent PD current data. As a result, a correlation between obtained statistical results and PD frequencies was examined. In most cases, the frequency of partial discharge decreased when the pressure increased. The amplitude of the partial discharges for negative polarity was more than that for positive polarity gas insulations. The partial discharge amplitudes of the pressure of 2 bar were mostly high compared to other pressures. This case demonstrated that SF6 had a maximum minimum character in terms of breakdown. The partial discharge frequency of a 1 % SF6 mixture was higher than that of pure SF6. It is thought that SF6 suppresses the discharge frequency, and statistical evaluations support the experimental results.